So we have two kitty cats. One, Cassie, is a 5 year old female who we've had since she was about 10 months old. She's only got 3 legs and she's a rescue but she's very affectionate and loving, and playful.
My cat, Sam, is only 9 months old, we got him about 2 months ago, and at first Cassie totally hated him. She wouldn't come in the same room, took to sulking all day in the garage, and hung around my mum ALL the time.
Now, she tolerates Sam but because he's so young, he wants to play all the time, and because Cassie's lively usually on her own, he tries to join in and she whacks him one, to which he smacks her back.
She doesn't mind sleeping next to him, eating with him, and sometimes they'll even get up close and sniff noses etc, but she suddenly will decide to hit him and it's upsetting.
It's never serious, neither of them get their hackles up or spit, they just yowl at each other and run around hitting each other.

Is there anything I can do to make it easier for Cassie to accept Sam, or to help them get along better? Or is it just a matter of waiting?

We do pay equal amounts of attention to them, they have seperate sleeping areas if they choose to use them, and use the toilet outside, so dunno what else to try...
[They're both fixed btw]

Sorry to hear about your cats. I haven't had any cats before so I'm not certain of ways to make them get along but perhaps negative/positive reinforcement could help like it does with dogs.

If they begin to fight you need to punish them. Maybe you could do this by taking away one of their toys or something similar. Likewise if they're playing together nicely then you need to reward them using treats or giving them a lot of attention. I know this works well for dogs but I don't know how well it would work for cats.

Also, if it's more the younger cat instigating this then perhaps give him more toys to be occupied with so he doesn't bother the older cat so much.

I hope this helped, take care and good luck with sorting your cats out

xD My cats do the same exact thing to each other, only they're only a year apart. I think they're just playing with each other to be honest, I mean if they aren't hissing or spitting at each other then playing is what it sounds like to me.

I know how awful it is to have cats who fight even if they aren't hurting each other. I have two cats (brother and sister) and even though they've known each other and been together their whole lives, they still fight like that. They've never hurt each other though, so I try not to worry about it too much.

If your cats are hurting each other and seem to be acting fine apart from occasionally hitting each other, they are probably okay and there's nothing much you can do. Sam might actually grow out of it as he gets older and become less interested in playing. But if it still really concerns you, a squirt from a water bottle should be enough to separate them and hopefully teach them not to fight.